Magnetism and electromagnetism revision

Question 1

unit for power

Answer 1

watt

Question 2

what do magnets repel and attract

Answer 2

repel and attract other magnets (depending on poles)
attract magnetic substances

Question 3

properties of magnetically hard materials

Answer 3

they’re resistant to demagnetising (become permanent magnets)

Question 4

properties of magnetically soft materials

Answer 4

they make an induced/ temporary magnets

Question 5

Rules of magnetic fields (+ lines)

Answer 5

-magnetic field lines represent a magnetic field
-always start at north pole and finish at south
-closer together lines are, stronger field is
-field lines never cross

Question 6

what’s a uniform magnetic field

Answer 6

north and south pole of diff magnets create a field where lines are evenly spaced and all going in smae direction

Question 7

electric current in a conductor…

Answer 7

… produces a magnetic field around it

Question 8

describe construction of an electromagnet

Answer 8

wrap coil (aka form a solenoid) around the magnetically soft material (e.g. iron). When the current is switched on, forced domains to line up and magnetizes it. When current is switched off, no longer magnetized.

Question 9

what happens to a charged particle (or current carrying wire) when moved in a magnetic field.

Answer 9

The wire/current carrier ur using experiences a force, as long as its motion isn’t parallel to the field. This is because of the two interacting fields - one from the wire itself and one from magnetic field it’s been placed in (e.g between two magnets)

Question 10

why does DC motor work

Answer 10

according to flemings left hand rule, when we reverse the current, we must also flip the direction of force (up to down vice versa). this ensures the motor keeps spinning and doesn’t just wobble

Question 11

purpose of a spit ring commutator

Answer 11

to reverse direction the current, used in DC motors

Question 12

how does a loudspeaker work

Answer 12

• current in the electrical current is varying and passes through the coil causing the coil to experience a force (inwards or outwards)
• The current is reversed repeatedly to reverse the force
• The greater the current the greater the force
• As the current and force vary, the coil the vibrates
• The coil is attached to a cone which also vibrates and causes the air molecules to vibrate
• This movement of the air molecules produces the pressure variations in the air needed for a sound wave producing compressions and rarefactions

Question 13

how can we increase speed of a dc motor (force of a wire)

Answer 13

increase current
increase magnetic field strength

Question 14

when is voltage induced in a conductor or a coil

Answer 14

when it moves thro a magnetic field or when a magnetic field changes thro it

Question 15

factors which increase size of induced voltage

Answer 15

speed of wire/coil/magnet
No_ turns on coils of wire
size of coils
strength of magnetic field

Question 16

how can electricity be generated by a magnet

Answer 16

by moving a magnet inside a coil of wire. the electricity is generated by the coil ‘cutting’ magnetic field lines

Question 17

what is a basic transformer made up of

Answer 17

a primary coil
a secondary coil
an iron core

Question 18

how does a transformer work

Answer 18

• An alternating current is supplied to the primary coil
• The current is continually changing direction meaning it will produce a changing magnetic field around the primary coil
• The iron core is easily magnetised, so the changing magnetic field passes through it and there is now a changing magnetic field inside the secondary coil
• This changing field cuts through the secondary coil and induces an alternating potential difference

Question 19

how can u change the size of an alternating voltage in a transformer

Answer 19

having different number of turns on the input and output sides

Question 20

what does each finger represent in flemings left hand rule

Answer 20

thumb - force
first - magnetic field
second - current

Question 21

purposes of step-up/ step-down transformers

Answer 21

• to increase p.d of electricity before it is transmitted across the national grid
• to lower high voltage electricity used in power lines to the lower voltage used in homes
• used in adapters to lower mains voltage to the lower voltages used by many electronic devices

Question 22

relationship between voltages and No_ turns for a transformer

Answer 22

input (primary) voltage / output (secondary) voltage = primary turns / secondary turns

Question 23

if we were to assume transformer was 100% efficient, equation between primary and secondary power

Answer 23

power primary = power secondary
primary voltage x primary current = secondary voltage x secondary current
Vp x Ip = Vs x Is